1. Field of the Invention
This invention relates to a fingerprint image input device, and more particularly to a thin and compact fingerprint image input device.
2. Description of the Prior Art
Conventionally, in a fingerprint image input device of the type mentioned, a fingerprint image is inputted to an image sensor after the brightness thereof is emphasized optically in order to obtain a high signal to noise ratio as disclosed, for example, in Shinichi Okumura and Masayuki Yamazaki, "Fingerprint Sensor by an Optical Path Separation Method Using Silicon Rubber," Autumn Meeting of the Applied Physics Society of Japan, 18P-N-14, 1992. FIG. 1 is a block diagram showing an example of a conventional fingerprint image input device. The conventional fingerprint image input device shown is constituted from the combination of prism 2, CCD camera 4, and light source 3. The fingerprint image input device functions in the following manner: Light emitted from the light source 3 is applied obliquely onto a finger 1. When the light comes upon a ridgeline of the fingerprint, total reflection of the light is not established between the prism 2 and the finger 1, causing the light to be scattered in all directions. As a result, the amount of light arriving at the CCD camera 4 through prism 2 is reduced. When light from the light source 3 does not come upon any ridgeline of the fingerprint, almost all of the light reaches the CCD camera 4 through prism 2 by total reflection. An image of finger 1 for which brightness is optically emphasized in this manner can be detected by CCD camera 4. It is to be noted that the incident angle of light to the finger 1 has an optimum range and hence the device must be designed so that the optimum range is always provided. If the device is designed so that light is applied onto the finger approximately normal thereto, the effect of optical emphasis in the brightness cannot be produced.
Meanwhile, as a conventional compact image input device, an image sensor of the full contact type such as disclosed, for example, in Japanese Patent Laid-Open No. Heisei 2-260568(1990) has been proposed and is adopted for use in a facsimile or an image scanner. FIG. 2 is an illustrative view showing the construction of the image sensor of the full contact type. A large number of sensor elements 102 are disposed one-dimensionally on a glass substrate 101 and combined with a collected fiber member 105 through a passivation film 103 and adhesive 104 so as to be opposed to an original 106. The image sensor functions in the following manner. Light emitted from a light source not shown is transmitted through the glass substrate 101 and collected fiber member 105 to the original 106. Part of the reflected light from the original 106 enters the collected fiber member 105 and is totally reflected repeatedly within the fibers until it comes to the sensor elements 102, where signal outputs are obtained from the sensor elements 102. By scanning the original 106 to read out signals continuously, a two-dimensional image can be inputted. Another image sensor of the full contact type has been proposed and is disclosed in Japanese Patent Laid-Open No. 3-154564(1991). FIG. 3 is an illustrative view showing the construction of the image sensor of the full contact type just described. The image sensor is constituted from the combination of a light emitting diode (LED) 107, a large number of sensor elements 102 disposed one-dimensionally on a transparent substrate 108, and a transparent glass case 109 into which optical fibers 110 are incorporated in such a manner as to be disposed obliquely relative to the sensor elements 102, and opposed to an original 106. The image sensor functions in the following manner. Light emitted from the light emitting diode (LED) 107 is transmitted through the glass case 109 to the original 106. Part of the light reflected by the original 106 enters the optical fibers 110 and is totally reflected repeatedly within the fibers until it comes to sensor elements 102, where signal outputs are obtained from the sensor elements 102. By scanning the original 106 to read out signals continuously, a two-dimensional image can be inputted.
However, with the conventional fingerprint image input device described above, since the prism 2 and CCD camera 4 are bulky, the optical path has a length of several cm or more. Accordingly, it is difficult to construct the device in a compact size.
Meanwhile, when it is attempted to construct a fingerprint image input device employing the conventional image sensors of the full contact type as described above, the following problems are involved. Firstly, a mechanical means is required for the conventional image sensor of the full contact type to scan a fingerprint. As a result, it is difficult to construct the device in a compact size. This problem could be eliminated if sensor elements can be disposed two-dimensionally to make up a two-dimensional image sensor. However, a second problem remains. This problem relates to the contrast of an output image. When a construction is employed wherein the original 106 is replaced by a finger 1 in the image sensor of the full contact type shown in FIG. 2, light is approximately normal to the finger 1 at the point of incidence. This does not allow the utilization of total reflection of light to emphasize the brightness of a fingerprint image. Thus, an image having a high contrast cannot be obtained.
On the other hand, the construction of the image sensor of the full contact type shown in FIG. 3 can be modified by moving the position of the light emitting diode 107 to apply incident light obliquely onto the finger 1 (refer to, for example, Japanese Patent Laid-Open No. 61-277252 [1986]). In this instance, while a possible modification can be made by using a technique equivalent to the conventional technique which makes use of total reflection, it is impossible to arrange sensor elements 102 and optical fibers 110 two-dimensionally, and therefore, it is difficult to provide a device of compact construction.